This invention relates generally to the combustion of hydrocarbon fuels and is particularly directed to an arrangement for directing a coal-water mixture fuel into a boiler for combustion therein.
A bulk liquid fuel is ideally subjected to an atomization process prior to its combustion for ensuring stable and efficient burning. During this atomization process, the liquid fuel is preferably broken up into the smallest possible droplet size to expedite its preheating and to ensure more probable and timely ignition of the fuel. In the case of a coal-water mixture fuel, atomization of the fuel preferably subdivides the bulk fuel to roughly the size of the coal particles suspended within the slurry to allow preheating and ignition to begin immediately for at least some of the coal particles. If the fuel slurry is not subdivided to this extent, the water tends to encase the coal particles and acts as a thermal insulator resulting in ignition delay and burnout of the fuel.
One form which the fuel atomizer/burner has taken is that of a rotating cup having a tapered inner surface extending from a first narrow end to a second expanded end of the cup. When a liquid fuel is provided to the inner surface of the tapered spinning cup at its narrow end, friction causes the fluid to rapidly attain the same tangential speed as the cup. The centrifugal force acting upon the fluid in a direction along the length of the cup causes the liquid to flow toward the lip, or rim, of the cup adjacent to its expanded end portion, which flow is oppposed by viscous drag. By varying the design and operation of the cup, e.g., its degree of taper, its angular velocity, etc., the thickness of the liquid fuel layer may be substantially reduced to promote the break-up of the fuel sheet which forms at and is discharged from the edge of the rotating cup into small droplets for improved fuel atomization and enhanced combustion.
The prior art discloses various fuel nozzle, or burner, arrangements for directing a liquid fuel into a combustion chamber for ignition and burning therein. One such approach is disclosed in patent application Ser. No. 564,127, filed Dec. 21, 1983 in the name of Larry W. Carlson, and assigned to the assignee of the present application. This fuel injection device includes a tubular housing axially mounted at a throat section thereof onto a combustion chamber. An axially adjustable pintle defines an annular chamber between the tubular housing and the combustion chamber. Axial adjustment of the pintle serves to constrict or enlarge the throat opening into the combustion chamber for regulating injection of the fuel into the combustion chamber. In cooperation with the fuel flow through the pintle, there is also provided means for injecting and swirling an oxidant or another fuel within the annular chamber. The discharges are such as to provide generally concentric impinging flows of fuel and oxidant into the combustion chamber. This arrangement provides an efficient means for controlling the tangential and radial directional components of oxidant flow entering the combustion chamber to permit shaping of the combustion plume within the combustion chamber to recirculate hot combustion gases for stable and symmetrical combustion conditions.
In general, prior art fuel burner arrangements for use with coal-water slurry fuels have suffered from various limitations. For example, the build-up of unburned carbon residue adjacent to the exit of the fuel delivery tube reduces the fuel flow within the system and degrades the combustion characteristics of the injected fuel. In addition, prior approaches have met with only limited success in attempting to atomize the fuel slurry to particle sizes on the order of the coal particles suspended within the slurry for preheating the coal and improving its combustion ignition. Failure to fully atomize the fuel slurry leads to the deposit of large collections of fuel particles within the combustion chamber and results not only in inefficient combustion but also leads to combustion chamber fouling as its operating characteristics are degraded by the unburned fuel, or slag, which collects therein.
The present invention thus represents an improvement over the prior art by providing a rotating cup burner arrangement for use with a coal-water mixture fuel which applies a thin, uniform sheet of fuel onto the inner surface of the rotating cup, inhibits the collection of unburned fuel on the inner surface of the cup, reduces the slurry to a collection of fine particles upon discharge from the rotating cup, and further atomizes the fuel as it enters the combustion chamber by subjecting it to the high shear force of a high velocity air flow.